An imaging device in which two imaging optical systems each including a wide-angle lens having a field angle wider than 180 degrees and an imaging sensor for imaging an image by the wide-angle lens are combined such that the respective object side lenses are opposed to each other, and the images by the respective imaging optical systems are synthesized to obtain an image in a solid angle of 4π radian is known (refer to Japanese Patent Publication No. 3290993).
Such an imaging device can simultaneously obtain image information in all directions; thus, it can be effectively used for a security monitoring camera or a car-mounted camera, for example. In recent years, it is required to downsize such an imaging device to be used as a portable imaging device.
Extremely accurate and fair image information can be obtained by using a small imaging device in a hand-held condition in report of news, for example.
It is preferable to deflect the light of the maximum image height away from the optical axis in the wide-angle lens having a field angle of 180 degrees or more for use in such an imaging device without using a sharp angle to be imaged on an imaging surface.
However, if the entire length of the wide-angle lens having a field angle of 180 degrees or more is reduced, it becomes necessary to drastically deflect the light beam in the periphery away from the optical axis. For this reason, a resolution in the peripheral portion of the imaging surface is reduced due to various aberrations.
It is necessary to gently deflect the light beam in the periphery for maintaining a high resolution in the periphery of the imaging surface. For this reason, the entire length of the lens is increased, and such an imaging device is not suitable for use in a hand-held condition.
Japanese Patent Publication No. 3290993 does not specifically describe a wide-angle lens.
Various wide-angle lenses having a wide field angle and a good performance have been conventionally proposed. Among them, the wide-angle lenses described in Japanese Patent Application Publication Nos. 2007-155977 and 2010-256627 specially have a good performance.
However, it is difficult to reduce the entire length of the wide-angle lens described in Japanese Patent Application Publication Nos. 2007-155977 and 2010-256627. If such wide-angle lenses are used as two wide-angle lenses for use in an imaging device, the size of the device is increased.
When using two wide-angle lenses as described in Japanese Patent Application Publication Nos. 2007-155977 and 2010-256627, it is difficult to reduce the distance between the optical axes of the two wide-angle lenses, and the overlapped portions of the images in the peripheral portions of the respective wide-angle lenses are misaligned to each other due to disparity. Thus, deterioration in an image is likely to remarkably develop in the jointed portion of the synthetic image.